A sealant film possessing an easy peel feature used for packaging materials in the manufacture of containers is required to satisfy both a hermetic seal and the easy peel feature. However, these two properties conflict with each other. Out of these two features, the hermetic seal is required to constantly yield a heat seal strength at least sufficient to protect the contents and further show a low degree of dependence of a heat seal strength on temperature so as to be compatible with a wide variety of substrates. On the other hand, with regard to the easy peel feature, the properties required from a viewpoint of preserving a good appearance and preventing contamination of the contents include not only the ability to leave no peel marks (threaded resinous residue) when the sealed surfaces are peeled apart, but also total inhibition of a phenomenon of resin threading while a heat sealing operation is required to be executed to the extent that the sealed surfaces can be readily separated by hand. Moreover, odor transfer from the film to foodstuff is preferably avoided wherever possible in the case of the food packaging sealant film that is offered to the food packaging uses.
So far many types of materials have been developed as a resin for the sealant film. For instance, U.S. Pat. No. 4,189,519 discloses an easy peel sealant material comprising ethylene-vinyl acetate copolymer (EVA) and poly-1-butene. Additionally, U.S. Pat. No. 4,666,778 discloses an easy peel sealant material comprising an ethylenic resin, poly-1-butene, and polypropylene. Nevertheless, according to studies conducted by the inventor of the present invention, the former is apt to be accompanied by transfer of acetic acid odor to the contents and the latter does not readily facilitate adjustment of the seal strength to an adequate degree.
Japanese Laid-open Patent Application No. 315,443/1989 discloses a composition for the easy peel sealant use comprising a poly-1-butene having an average molecular weight of not less than 500,000 and low-density polyethylene. The composition is, however, usable only with limited types of processing techniques due to the high molecular weight of its poly-1-butene content and its low melt flow rate, besides having narrow tolerances in terms of controllability of the seal strength. Although the “Research Disclosure” magazine (issue No. 38433) refers to an easy peel laminated body comprising poly-1-butene and polyethylene polymerized with the metallocene catalyst, this literature fails to give any description regarding the phenomenon of resin threading. Furthermore, Japanese Laid-open Patent Application No. 337,829/1998 describes that a composite film constructed of a seal layer comprised of low-density polyethylene and poly-1-butene and a substrate layer produced from polyethylene resin gives good hermetic seal and openability.